The YAK40 is a most interesting corporate jet: there are loading stairs in the rear. Where on earth do the avionics, hydraulic system, wiring and other stuff go? Wouldn't the rear door take up a lot of space? Isn't it sort of a waste of money to put a rear door since corporate jets carry only about 10-20 people on average? I guess the Russians have little patience when boarding or leaving a plane!

1) The Yak40 is NOT a corporate jet. It is an airliner that seats from 24 to 40 pax depending on config.
2) It was designed to operate from semi-prepared (gravel) or grass airfields.
3) It was designed for internal Russian feeder services, hence it often operated multi stop, bus like route from very small airports, hence having built in airstairs was convient
4) Built in airstairs were once very common and can be found on the Caravelle, B727, DC9 and B737-200 (not rear airstairs), also BAC 1-11? & HS Trident?

The BAC 1-11 certainley had rear airstairs,I used them on a BIA 1-11-400 on a flight from Gatwick to Naples, but I am quite sure the Trident didn't have them, although it did have to doors on each side forward of the wing.